Last() throws an exception when applied on a concatenation of a collection with two empty collections

[AnnaVel]

Description

Hi, we've run into an issue with .NET 9, preview 2 onwards. If you have an enumerable with some elements and you concatenate it with other two enumerables which are empty the Last() method, applied to the result concatenation, throws an InvalidOperationException 'Sequence contains no elements'.

Reproduction Steps

IEnumerable<int> results = new List<int>() { 1, 2, 3 }.Concat(new List<int>()).Concat(new List<int>());

bool any = results.Any(); // true
int count = results.Count(); // 3
int last = results.Last(); // 'Sequence contains no elements.'

Same code in a project: https://github.com/AnnaVel/ConcatenatedLast

Expected behavior

Last() should return 3.

Actual behavior

Last() throws an exception.

Regression?

Yes. This does not reproduce in .NET 9 preview 1, but reproduces in .NET 9 preview 2 and later.

Known Workarounds

One could just use collection.ElementAt(collection.Count() - 1).

Configuration

It is not likely this is a configuration-specific problem.

Other information

No response

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[andrewjsaid]

The reason for this bug is that in Concat.SpeedOpt we have loop through the tail using this

while ((node = node!.PreviousN) is not null)

runtime/src/libraries/System.Linq/src/System/Linq/Concat.SpeedOpt.cs

Line 338 in 0d86380

while ((node = node!.PreviousN) is not null);

Which is defined as

private ConcatNIterator<TSource>? PreviousN => _tail as ConcatNIterator<TSource>;

runtime/src/libraries/System.Linq/src/System/Linq/Concat.cs

Line 152 in 0d86380

private ConcatNIterator<TSource>? PreviousN => _tail as ConcatNIterator<TSource>;

However the "base case" of the tail is a Concat2Iterator``1 so null is returned and so the "base source" isn't inspected.